1. Field of the Invention
The invention relates to a diagnostic system and diagnostic method for an internal combustion engine including a variable mechanism that changes a working angle of an intake valve.
2. Description of Related Art
In recent years, an internal combustion engine that is mounted on a vehicle executes catalyst early warm-up control for raising an exhaust gas temperature by retarding ignition timing and increasing an intake air amount as compared to that during normal idle operation in order to suppress a decrease in idle rotation speed due to the retardation of the ignition timing at a cold start. An increase or reduction in the intake air amount is a cause of a change in the amount of heat that is supplied to the catalyst, and an early warm-up of the catalyst cannot be achieved when the intake air amount is small. Therefore, a diagnostic system described in Japanese Patent Application Publication No. 2003-201906 (JP 2003-201906 A) diagnoses an abnormality when an accumulated value of the intake air amount during execution of catalyst early warm-up control is smaller than or equal to a predetermined determination value.
There is an internal combustion engine including a variable mechanism that changes the working angle of the intake valve. In such the internal combustion engine, the flow rate of air that is taken into a cylinder is raised by narrowing the working angle of the intake valve, thus making it possible to facilitate mixing of fuel and air. When mixing of fuel and air is facilitated by narrowing the working angle of the intake valve, combustion takes place in a desired manner, so a misfire is hard to occur even when the ignition timing is retarded. Therefore, in an internal combustion engine including a variable mechanism that changes the working angle of an intake valve, catalyst early warm-up control may be executed in a state where the working angle of the intake valve is narrowed.